1. Field of the Invention
The invention relates generally to a method of manufacturing a semiconductor device, and more particularly to, a method of manufacturing a semiconductor optical device of a planar buried heterostructure (hereinafter called xe2x80x98PBHxe2x80x99) type in which a mode converter is integrated.
2. Description of the Prior Art
A semiconductor optical amplifier is usually used as a functional device of a wavelength division multiplexing (WDM) optical communication network. There has recently been made a lot of research and development on the semiconductor optical amplifier. The type of the semiconductor optical amplifier can be functionally classified into a mode converter for reducing the optical loss caused when a semiconductor optical device and an optical fiber are coupled, and one in which current is injected into an active layer for the purpose of the gain of a semiconductor optical device.
The mode converter (or mode expander) is to reduce the optical loss that is caused when the semiconductor optical device and the optical fiber are coupled. The optical loss is generated when a mode of the optical fiber of about 8xcx9c10 xcexcm2 in the mode size and a mode of the optical device of about 1xc3x970.5 xcexcm2 in the mode size due to a large refractive index of the semiconductor is mismatched. The above optical loss requires an optical alignment of a higher degree upon an optical packaging, and also greatly affects the characteristics of an active optical device due to a reflecting light caused in the cross section of the optical device. Therefore, research has recently been made on a mode converter integrated optical device in order to reduce the cost and accomplish single integration between the optical devices.
Next, in case of the one in which current is injected into the active layer for the purpose of the gain of the semiconductor optical device, the two waveguides are different in the width and have a vertical type in space, in the optical device having two waveguides as a basic structure. Therefore, there is a problem in technically regrowing an electrical isolation layer and in repeatability, etc. accordingly.
In order to solve these problems, U.S. Pat. No. 6,025,207 discloses a method of simplifying the process by improving the electrical isolation layer. FIG. 1 is a cross-sectional view of an optical device of a buried ridge structure (hereinafter, called xe2x80x98BRSxe2x80x99) disclosed in the U.S. Pat. No. 6,025,207.
Referring now to FIG. 1, the optical device of BRS type in the U.S. Pat. No. 6,025,207 includes, as basic components, an optical waveguide layer 102 for integration and an active layer 104. A p-InP layer 106 for current injection is regrown on the entire structure. A proton 108 is injected into regions except for the active layer 104 using the ion implanter and current is injected into the active layer 104 in order to simplify the process for integration. Further, an InGaAs layer 110 into which a p type impurity is doped and having a high conductivity as an ohmic layer is formed. An electrode 112 is formed to complete the optical device. However, as there exists the difference in the width of current injection and in the width of the active layer, electrons are leaked to the p-InP layer 106 when high current is injected. As the InP layer 106 into which a p type impurity is doped is regrown on the first waveguide in regions except for the active layer 104, the propagation loss occurs. There is a problem that the propagation loss degrades the characteristic of an active optical device. In FIG. 1, unexplained reference numerals 100, 101 and 103 indicate a substrate, a buffer layer, and a medium layer into which a n type impurity of a high concentration is doped, respectively.
U.S. Pat. No. 5,863,809 discloses a method of reducing this propagation loss. FIG. 2 is a cross-sectional view of an optical device of a buried ridge structure (BRS) type disclosed in the U.S. Pat. No. 5,863,809.
Referring now to FIG. 2, waveguide layers 202 and 204, and an active layer 206 are first grown. A waveguide of the active layer 206 is then formed using photolithography and etching. An undoped layer 210 is regrown on the entire surface to reduce the propagation loss.
Further, the undoped layer 210 on the active layer 206 is etched by means of photolithography so that current can be injected into only the active layer 206. InP layers 214 and 212 being an electrical isolation layer and being doped by p/n/p type, are formed in regions except for the active layer 206. The U.S. Pat. No. 5,863,809 can obtained a current isolation characteristic using the undoped InP layer 210. However, the patent has a disadvantage that current is distributed due to the thickness of the undoped layer 210. In addition, as photolithography and etching process are performed several times, repeatability is degraded and the yield is thus reduced. Considering that the yield of the optical devices is very lower than that of electronic devices, reduction in the yield must be solved. In FIG. 2, unexplained reference numerals 207, 208, 216, 218 and 220 indicate a p-InP layer, an etch stop layer, an upper p-InP layer, a contact layer and a contact pad, respectively.
The present invention is contrived to solve the above problems and an object of the present invention is to provide a method of manufacturing a semiconductor optical device having a PBH (planar buried heterostructure) type of a good characteristic, capable of reducing the propagation loss, simplifying the process using a self-aligned method, and increasing the repeatability and yield.
In order to accomplish the above object, a method of manufacturing a semiconductor optical device according to the present invention, is characterized in that it comprises the steps of sequentially forming a first waveguide layer, a first clad layer, a second waveguide layer and a second clad layer on a semiconductor substrate; depositing a first hard mask on the second clad layer and then forming a first hard mask pattern having a taper shape at both ends; etching the second clad layer and the second waveguide layer using the first hard mask pattern as an etch mask; selectively growing an undoped InP layer at regions from which the second clad layer and the second waveguide layer are etched using the first hard mask pattern as a selective mask and then patterning the undoped InP layer; removing the first hard mask pattern; forming a second hard mask pattern the width of which is smaller than the width of the first hard mask pattern on a result from which the first hard mask is removed; and etching the undoped InP layer, the second clad layer, the second waveguide layer, the first clad layer and the first waveguide layer using the second hard mask pattern as an etch mask to simultaneously form a second waveguide layer and a first waveguide layer having different widths.
The method of manufacturing a semiconductor optical device according to the present invention, is characterized in that it can further comprises the steps of, after the step of etching the undoped InP layer, the second clad layer, the second waveguide layer, the first clad layer and the first waveguide layer using the second hard mask pattern as an etch mask, forming an electrical isolation layer at regions where the undoped InP layer, the second clad layer, the second waveguide layer, the first clad layer and the first waveguide layer are etched; removing the second hard mask pattern; forming an upper clad layer on a result from which the second hard mask pattern is removed; depositing an electrically conductive layer on the upper clad layer and then patterning the electrically conductive layer in order to inject current into the second waveguide layer; forming a silicon nitride film on a result in which the electrically conductive layer is pattern and then etching the silicon nitride film in order to electrically connect the electrically conductive layer and an electrode, thus forming an opening portion; and forming an electrode electrically connected to the electrically conductive layer.
The electrical isolation layer is formed by growing an InP layer in a p/n/p type.
The first waveguide layer is formed to be a n-InGaAsP layer, the second waveguide layer is formed to be an undoped InGaAsP layer, the first clad layer is formed to be a n-InP layer, and the second clad layer is formed to be a p-InP layer.
An InGaAsP layer is further formed at lower and upper sides of the second waveguide layer for separate confinement heterostructure and uniform gain depending on polarization.
The widths at the central portion and edge of the second hard mask pattern are different, the width at the edge of the second mask pattern is wider than the width at the central portion of the second mask pattern, the width of the central portion of the second mask pattern is narrower than the wide of the fist hard mask and the boundary at the central portion and the edge of the second mask pattern is tapered
The etching process using the second hard mask pattern as an etch mask includes first performing a dry etching process to etch by a given depth and then performing a wet etching process.
The first and second hard masks are formed to be a silicon nitride film.